Remote-controlled bots have managed to get accurate radiation measurements from inside two reactor buildings at Fukushima's crippled Daiichi plant.

To prevent dangerous radiation exposure to workers, TEPCO deployed remote-controlled robots that had been sent from the US in number 1 and number 3 reactor buildings where radioactivity is still too high for staff to venture inside.

Sent in on Sunday, the readings taken by the two caterpillar-wheeled PackBot robots to measure radiation levels on the ground floor of the reactor buildings showed the maximum radiation level inside the No. 3 reactor building at 57 millisieverts per hour, while in the No.1 building they measured 10 to 49 millisieverts per hour.

According to TEPCO any exposure to the levels found in both plants would exceed the emergency safety limit for nuclear power plant workers, set at 250 millisieverts per year.

The PackBots are made by US based iRobot, the same company behind the vacuum Roomba. They are equipped with radiation and temperature sensors and can open unlocked doors. The two at the plant were remotely operated via a fibre optic cable.

Oxygen densities in both buildings were found to be around 21 per cent, good enough for normal work. But the high levels of radiation means that it is going to be near impossible for human workers to re-enter the site for now, says a TEPCO spokesman.

Normal radiation at the site before the tsunami were in the region of 0.01 millisieverts per hour, he said. Officials had deployed a Japanese robot earlier last month with similar capabilities but it was unclear why Japan was now using the US robots, which arrived at Fukushima over two weeks ago. According to the Tepco spokesman it had taken that long to train someone how to operate the complex machines.